BCD to 7-segement display decoder

Here is a program for BCD to 7-segment display decoder. The module takes 4 bit BCD as input and outputs 7 bit decoded output for driving the display unit.A seven segment display can be used to display decimal digits.They have LED or LCD elements which becomes active when the input is zero.The figure shows how different digits are displayed:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity test is
port (
      clk : in std_logic;
        bcd : in std_logic_vector(3 downto 0);  --BCD input
        segment7 : out std_logic_vector(6 downto 0)
    );
end test;
--'a' corresponds to MSB of segment7 and g corresponds to LSB of seg7.
architecture Behavioral of test is

begin
process (clk,bcd)
BEGIN
if (clk'event and clk='1') then
case  bcd is
when "0000"=> segment7 <="0000001";  -- '0'
when "0001"=> segment7 <="1001111";  -- '1'
when "0010"=> segment7 <="0010010";  -- '2'
when "0011"=> segment7 <="0000110";  -- '3'
when "0100"=> segment7 <="1001100";  -- '4' 
when "0101"=> segment7 <="0100100";  -- '5'
when "0110"=> segment7 <="0100000";  -- '6'
when "0111"=> segment7 <="0001111";  -- '7'
when "1000"=> segment7 <="0000000";  -- '8'
when "1001"=> segment7 <="0000100";  -- '9'
 --nothing is displayed when a number more than 9 is given as input. 
when others=> segment7 <="1111111";
end case;
end if;

end process;

end Behavioral;

   If you want a decimal number to be displayed using  this code then convert the corresponding code into BCD and then instantiate this module for each digit of the BCD code.

Here is a sample test bench code for this module:

LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;

ENTITY test_tb IS
END test_tb;

ARCHITECTURE behavior OF test_tb IS
   signal clk : std_logic := '0';
   signal bcd : std_logic_vector(3 downto 0) := (others => '0');
   signal segment7 : std_logic_vector(6 downto 0);
   constant clk_period : time := 1 ns;
BEGIN
   uut: entity work.test PORT MAP (clk,bcd,segment7);
   clk_process :process
   begin
                clk <= '0';
               wait for clk_period/2;
                clk <= '1';
                wait for clk_period/2;
   end process;  
   stim_proc: process
   begin             
     for i in 0 to 9 loop
           bcd <= conv_std_logic_vector(i,4);
          wait for 2 ns;
     end loop;
   end process;

END;